1. Field of the Invention
This invention relates to a method for forming a stable, oxide-free silicon surface that inhibits re-growth of an oxide layer for long periods of time in a clean room environment.
2. Description of the Related Art
Clean semiconductor surfaces are a key factor in preparing integrated circuits in high yields. There are two major types of contamination which occur: films and particulates. Particulates are materials that have readily defined boundaries, while films (for example, native oxide on a bare silicon surface) are layers of material on the surface of the wafer.
It is important to minimize or eliminate both films and particulates on the surface of the silicon wafer in order to optimize integrated circuit yields. Prior to epitaxial deposition and diffusion processes on bare silicon surfaces, particularly for processes conducted at less than about 850xc2x0 C., it is important to have a clean oxide-free silicon surface.
Particulates and films may be removed through cleaning. The standard cleaning method often involves one or more forms of an RCA cleaning procedure. The RCA Standard-Clean-1 (SC-1) procedure uses a mixture of hydrogen peroxide, ammonium hydroxide, and water heated to a temperature of about 70xc2x0 C. The SC-1 procedure dissolves films and removes Group I and II metals. The Group I and II metals are removed through complexing with the reagents in the SC-1 solution.
The RCA Standard-Clean-2 (SC-2) procedure utilizes a mixture of hydrogen peroxide, hydrochloric acid, and water heated to a temperature of about 70xc2x0 C. The SC-2 procedure removes the metals that are not removed by the SC-1 procedure. If an oxide-free surface is required, the silicon wafer is dipped into an aqueous solution of hydrofluoric acid to etch away the oxide layer and, theoretically, obtain hydrogen termination. There are a large number of variations on RCA clean and hydrofluoric acid dips.
After cleaning, wafers are typically stored for a period of time before further processing. . A native oxide tends to form on the oxide-free silicon surface almost immediately after exposure to air or moisture. Further, silicon-fluorine and silicon-carbon bonds are often observed on the silicon surface after cleaning. The fluorine and carbon contamination on the surface may be detrimental to the thermal budget and/or the quality of the layer to be grown or deposited on the surface of the wafer.
If the silicon wafer is dipped in hydrofluoric acid as the last cleaning step (also known as an xe2x80x9cHF lastxe2x80x9d step), the surface of the silicon is typically terminated mostly with a monolayer of hydrogen, largely Sixe2x80x94H bonds. The hydrogen-terminated surface prevents oxidation better than without any termination. However, the surface of a silicon wafer after an HF last treatment normally starts to reoxidize within about 20 minutes after the original oxide layer was removed, quickly forming a new 5 xc3x85 to 7 xc3x85 thick oxide layer on the surface of the silicon wafer. Even with the best cleaning processes currently known, a layer of native oxide forms within 48 hours, and, often the wafers cannot be further processed within that time. This will mandate a new HF dip if an oxide-free surface is required for the next process step.
Carbon or fluorine termination can better prevent re-oxidation, but they introduce other problems, such as contamination or difficulty in removing the termination prior to subsequent processing. Hydrogen termination can advantageously be removed at about 500xc2x0 C.
In an HF last, when the oxide layer is removed from the surface with a hydrofluoric acid solution as the final step in the cleaning procedure, the wafer surface has a tendency to have high levels of particles due to: 1) exposure to contaminants in the solution; 2) exposure to air at the air/liquid interface; 3) deposition of particles during the drying process; and 4) exposure to air during the time between the drying step and the time that the silicon wafer is placed in an inert environment.
Oxide regrowth on the surface of the cleaned wafer can be inhibited for longer periods of time by storing the wafers in an inert environment, such as a nitrogen or argon atmosphere. However, even the exposure to air during the time period between the time that the wafer is removed from the cleaning bath and the time that the wafer is placed in the inert atmosphere can lead to oxide regrowth. Expensive special hardware is thus required to ensure that the cleaned wafers are transported and stored in inert, purged environments from the time of cleaning to the next process.
Accordingly, there is a need for a method of cleaning a silicon surface in which the cleaned surface has enhanced stability against oxidation in a clean room environment, without the need for special transport and storage hardware.
In accordance with one aspect of the invention, a method is provided for forming a stable oxide-free silicon surface. The method includes cleaning a silicon surface. After cleaning, the silicon surface grows a native oxide of less than 1 xc3x85 after exposure to air for more than about 3 days.
In the illustrated embodiments, a standard APM clean is followed by a dilute HF dip to etch oxide grown by APM clean. The dilute HF solution is then rinsed away in situ, and the substrate (e.g., silicon wafer) is spin-dried under heated, ionized, high purity purge gas (i.e., N2, Ar). Advantageously, the dilute HF etch and rinse steps employ ultrapure water with a resistivity at 25xc2x0 C. of greater than 16 Mxcexa9-cm and less than 10 ppb total organic carbon, less than 10 ppb dissolved silica, and less than 500 ppb dissolved oxygen. The process has been shown to demonstrate stability against significant native oxide regrowth for 8 days.
In accordance with another aspect of the invention, a method is provided for forming a stable oxide-free silicon surface of a substrate. The method includes cleaning said silicon surface and chemically growing an oxide. This oxide is then etched with hydrofluoric acid. The substrate is then rinsed and dried. The etching, rinsing and drying processes add fewer than 0.032 particles/cm2 having a size larger than 0.12 xcexcm to the silicon surface. For example, fewer than 10 particles ( greater than 0.12 xcexcm) are added to a 200 mm wafer. In the illustrated embodiments, these processes add no net particles greater than 0.12 xcexcm.
In accordance with another aspect of the invention, a bare silicon wafer is provided with a predominately hydrogen termination that is stable enough to limit oxidation to less than about I A upon exposure to a clean room environment for greater than 3 days.